Measurement of the relative amounts of gas contained in a sample mixture is often made by a gas chromatograph which includes means for causing a stream of carrier gas to flow through a column and a detector in sequence and means for injecting a known quantity of the mixture of gases to be quantified into the stream of carrier gas just before it enters the column. Ideally, each gas in the sample emerges from the column at different times so that, at any one time, the gas flowing into the detector is either all carrier gas or a combination of carrier gas and one of the gases of the sample mixture. The detector functions by producing a signal related to the change in the intensity of a given characteristic of the gases flowing through it. The intensity of the characteristic of the carrier gas is generally either greater or less than the intensity of the same characteristic of the gases in the sample mixture. Thus, as each sample gas passes through the detector, the output signal varies from the value it has when the detector is full of carrier gas, the amount of variation depending on the concentration of the sample gas.
One of the most widely used detectors is the thermal conductivity detector. It is comprised of a block having a cavity within it, a filament suspended in the cavity and ports at either end of the filament, one of which is connected to the end of the column from which the gases are eluting. Current is passed through the filament so as to heat it, and means are generally provided for maintaining the block and therefore the walls of the cavity at a fixed temperature that is less than the temperature of the filament. The output signal of the detector corresponds to the variation in voltage applied to the filament or the current flowing through it that are required to keep the filament at a given temperature or resistance. The temperature of the filament depends on the rate at which heat can flow from it to the walls of the cavity. Nearly all of the heat flows by conduction through the gases between the filament and the walls of the cavity. Because of its inertness and the fact that its thermal conductivity is greater than that of all gases except hydrogen, helium is generally used as a carrier gas.
Measuring the relative amount of hydrogen in a sample mixture is very difficult if helium is used as the carrier gas because the thermal conductivity of hydrogen is only slightly greater than that of helium, and also because the thermal conductivity of a mixture of hydrogen and helium decreases as the percentage of hydrogen in helium is gradually increased, rather than increasing as might be expected. After reaching a minimum value, the thermal conductivity of the mixture increases so as to pass through the thermal conductivity of helium alone so that only percentages of hydrogen less than that producing the minimum thermal conductivity can be measured. Depending on the temperature involved, the minimum thermal conductivity occurs at a very low percentage of hydrogen, usually less than 15%. Thus, if a sample mixture contains too much hydrogen, it must be diluted sufficienly to make the percentage of hydrogen in helium less than that which produces the minimum thermal conductivity of the mixture. A chromatogram representing hydrogen is on the opposite (-) side of the baseline from the chromatogram for other gases. Similar phenomena occur in measuring the relative amount of deuterium when helium is used as the carrier gas.
Another factor that interferes with the measurement just described in catalytic disassociation of hydrogen that takes place on the surface of the filament, i.e., the formation of two atoms H for each hydrogen molecule. Because this is an endothermic process, it draws heat from the filament so as to cause an error in the signal produced by the detector. Whereas these errors are small, it must be remembered that the signal is also small. Further difficulty arises from the fact that the error varies in such manner as to make compensation impossible.
The usual way of avoiding these difficulties is to employ a separate injector, column and detector for measuring hydrogen and using nitrogen or argon as the carrier gas. Because the thermal conductivity of the hydrogen is significantly different from either nitrogen or argon, the sensitivity is increased, thus reducing but not eliminating the errors introduced by catalytic disassociation. Of greater importance, however, is the fact that the thermal conductivity of the mixture of hydrogen and either nitrogen or argon increases with the percentage of hydrogen so that there is no minimum value as previously described.